Structural validity of the Brazilian version of the Western Ontario and McMaster Universities Osteoarthritis Index among patients with knee osteoarthritis

BACKGROUND: The original structure of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) has been contested in several languages. OBJECTIVE: To assess the structural validity of the Brazilian version of WOMAC among patients with knee osteoarthritis. DESIGN AND SETTING: Structural validity study conducted at physiotherapy clinics and primary healthcare units. METHODS: The study included males and females aged 40 to 80 years who were all native Brazilian Portuguese speakers, with knee pain in the previous six months and a diagnosis of knee osteoarthritis. We used exploratory factor analysis (EFA) followed by confirmatory factor analysis (CFA) with implementation of a polychoric matrix and the robust diagonally weighted least squares (RDWLS) extraction method. The adequacy of the model was assessed using the following fit indices: root mean square error of approximation (RMSEA), comparative fit index (CFI), Tucker-Lewis index (TLI), standardized root mean square residual (SRMR) and chi-square/degree of freedom (DF). RESULTS: 203 patients with knee osteoarthritis were included. The model proposed in this study with two factors, i.e. “pain” (items 1, 2, 3 and 4) and “physical function” (items 10, 11, 16, 17, 18, 19, 21 and 22), showed adequate fit indices in CFA: chi-square/DF = 1.30; CFI = 0.976; TLI = 0.970; RMSEA = 0.039; and SRMR = 0.070. The factorial loads ranged from 0.68 to 0.76 for the “pain” domain and 0.44 to 0.62 for the “physical function” domain. CONCLUSION: The Brazilian version of WOMAC with two domains, i.e. “pain” (four items) and “physical function” (eight items), presents the best structure.


INTRODUCTION
The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is a patientbased self-report instrument that was created and validated in English in 1988, to measure pain, joint stiffness and physical function among patients with hip or knee osteoarthritis. This initial study involved face, content and construct validity, reliability and responsiveness. 1 Since its creation, WOMAC has been translated, adapted and validated for use in several other languages, such as German, Spanish, Japanese, Swedish and Arabic. 2 There is also a Brazilian Portuguese language version but, curiously, the study in which the translation, cross-cultural adaptation and validation were performed was not published in a peer-reviewed scientific journal (it was a master's dissertation). However, adequate values for reliability and construct validity were identified. 3 In that version of WOMAC in Brazilian Portuguese, the structural validity of the questionnaire was not ascertained. In other languages, some studies have investigated the structural validity of WOMAC by means of factor and Rasch analysis. According to a systematic review published in 2015, 2 factor analysis was conducted in five studies and variation from three to seven in the number of WOMAC domains was observed. Bilbao et al. 4 highlighted that the Spanish structure of WOMAC with three domains and 24 items was inadequate and proposed a short version with two domains and 11 items, through using confirmatory factor analysis. Rothenfluh   Also using Rasch analysis, Davis et al. 6 proposed a new English version of WOMAC with two domains and 17 items.
Thus, considering the different investigations conducted and the different scientific conclusions reached regarding the structure of WOMAC, our study was justified by the need to identify whether the original structure of WOMAC, as used in its translation into the Brazilian Portuguese language, is adequate.

OBJECTIVE
The aim of this study was to assess the structural validity of the Brazilian version of WOMAC, among patients with knee osteoarthritis.

Ethical aspects
This study was based on secondary analysis on data from previous studies. 7

WOMAC
This study used the WOMAC version with Likert scale responses.
As in the study conducted by Fernandes, 3 the Brazilian Portuguese version has three domains, namely: "pain" domain with five items (items 1, 2, 3, 4 and 5); "stiffness" domain with two items (items 6 and 7); and "physical function" domain with 17 items (items 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 and 24). For each item, there are five possible answers, ranging from 0 to 4. The score for each domain is calculated as the simple sum of the items answered: in the "pain domain", the score ranges from 0 to 20; in the "stiffness" domain, the score ranges from 0 to 8; and in the "physical function" domain, the score ranges from 0 to 68 points.
In the original version, the reliability found was considered adequate, with an intraclass coefficient correlation ranging from 0.73 to 0.97. Regarding the construct validity, there was an adequate correlation between the WOMAC domains and the Visual Analogue Scale, Health Assessment Questionnaire and Lequesne Algofunctional Index, with correlation magnitudes ranging from 0.425 to 0.935.

Statistical analysis
To identify the best WOMAC structure for its version in the In the present study, values greater than 0.90 were considered adequate according to the CFI and TLI, and values less than 0.08 were considered adequate according to the RMSEA and SRMR.
Values below 3.00 were considered adequate in interpreting the chi-square/DF data. 18,19 In CFA, factorial loads greater than or equal to 0.40 were considered adequate for the domain. The Akaike information criterion (AIC) and Bayesian information criterion (BIC) were used to compare the models, and the lowest value was considered to be the most appropriate.

RESULTS
This study included 203 patients with knee osteoarthritis. The personal and clinical characteristics are described in Table 1. In our sample, most of the patients were elderly, female and overweight.
The EFA was carried out to explore and identify the structure of the Brazilian version of WOMAC. By means of parallel analysis, two factors were identified: the "pain" domain (items 1 to 5) and the "physical function" domain (items 6 to 24). This WOMAC structure was called Model 1. The EFA with parallel analysis presented suitable fit indices: KMO = 0.75 and Bartlett's test with P < 0.001. Figure 1 presents the scree plot of this parallel analysis with the two factors defined.

DISCUSSION
Our study revealed that the most suitable structure for WOMAC in Brazilian Portuguese has two domains: four items in the pain domain (items 1, 2, 3 and 4) and eight items in the physical function domain (items 10, 11, 16, 17, 18, 19, 21 and 22). The structural validity of WOMAC has been tested in several studies. According to a systematic review conducted by Gandek,2 EFA was carried out on four studies, [20][21][22][23] and the number of factors (domains) retained for the English and Chinese versions of WOMAC were 3, 4, 5 and 7. This number of domains was higher than the two found in the present study.
We emphasize that our study used parallel analysis as the factor retention method, whereas the abovementioned EFA used other methods for such purposes. Currently, parallel analysis is considered to be a more adequate and robust method for identifying the number of factors in a questionnaire. 24,25 Another positive point of the present study is that we implemented factor analysis based on a polychoric matrix and we used RDWLS as an extraction method. These implementations are appropriate and should be used for ordinal categorical data, as in the case of the Likert scale (0, 1, 2, 3 and 4). 11,12 In addition to the studies cited in the systematic review conducted by Gandek, 2 the original WOMAC structure with three domains and 24 items has been rejected by other authors who used the Rasch analysis. Davis et al. 6 included patients before and after total hip or knee arthroplasty and identified two domains as the best WOMAC structure in English: pain (three items) and physical function (14 items). Another study that investigated the structure of the German version of WOMAC included patients with femoroacetabular impingement and hip osteoarthritis. These authors established a one-dimensional structure for WOMAC with 12 items as the appropriate option. 5  In addition, we performed a comparison between different structures for WOMAC; the structure that we proposed presented better fit indices for the model compared with the original structure of WOMAC (three domains; 24 items) and the structure proposed by Bilbao et al. 4 Our data show that the original version of WOMAC, traditionally used in Brazil by researchers and clinical professionals,  should be replaced by the short version presented here. WOMAC is the questionnaire most used to track and identify the signs and symptoms of patients with osteoarthritis. It is an adequate tool for following the clinical changes among patients in the light of therapeutic interventions. We firmly believe that our results, based on factor analysis and model comparison, should serve as a basis for a new understanding of WOMAC and its "pain" and "physical function" domains.
Our study has some limitations. Only structural validity was considered in the present study. Our sample consisted of patients with knee osteoarthritis, and hip osteoarthritis patients were not included. Other important psychometric properties need to be evaluated through future studies, such as reliability and construct validity (correlation with other instruments and questionnaires that measure pain and function). 9

CONCLUSION
The Brazilian version of WOMAC with two domains, i.e. "pain" (four items) and "physical function" (eight items), presents the best structure.